The present invention relates to a semiconductor memory device and, more particularly, to a dynamic random access memory device (DRAM) having memory cells each composed of one transistor and one storage capacitor.
In accordance with increase in memory capacity of a DRAM, an area which is able to be allotted to each memory cell, i.e. a cell size, is required be made small. Therefore, such a memory cell is required that can enlarge the capacitance value of the storage capacitor with a small cell size.
As one of memory cells for that purpose, the so-called stacked-type memory cell is known in the art and shown in FIGS. 1A to 1C. FIG. 1A is a plan view of the stacked memory cell and FIGS. 1B and 1C are cross sectional views along lines A--A and B--B shown in FIG. 1A, respectively. In this memory cell, the reference numeral 201 denotes a silicon substrate and 204 denotes a word line serving as the gate electrode of a cell transistor. The reference numeral 209 denoted a bit line contact for connecting a bit line 210 and one of the drain region 203a of the cell transistor, 205 denoting a capacitor contact for connecting a storage electrode 206 and the source region 203b, 207 denoting a dielectric film, 208 denoting a cell plate electrode 208, and 211 denoting an interlayer insulating film made of silicon oxide. The storage capacitor is thus composed of the electrodes 206 and 208 and the dielectric film 207 formed therebetween and stacked over the word line 204. Accordingly, the memory cell presents a relatively large storage capacitance value with a small cell size.
As shown in the drawings, the isolation between the adjacent memory cells is performed by the so-called selective-oxidation insulating film 202. This insulating film 202 is formed by selectively covering the substrate 201 with a oxidation-withstand film and then oxidizing the substrate 201 by using the oxidation-withstand film as a mask. The parts of the substrate 201 which are not covered with the oxidation-withstand film are thereby converted into an oxide film. Thus, the selective-oxidation insulating film 202 is formed. Each of the regions surrounded by the selective-oxidation insulating film 202 are called an "active region". That is, each of the active regions 212 is defined by the selective-oxidation insulating film 202.
Each active region 212 is generally designed to be formed in a rectangular shape. In accordance with the miniaturization of the memory cell, however, each active region 212 is actually formed in a rectangular shape having a rounded corner, as well known in the art. As also known in the art, the selective-oxidation insulating film 202 has the so-called bird's beak portion cutting into the active region 212. For these reasons, the effective area of each active region 212 is made small, so that the capacitor contact 205 happens to be formed on and thus in contact with the selective-oxidation insulating film 202. The leakage of the charges stored in the storage capacitor thereby occurs to destroy the stored data.